Local area network

ABSTRACT

A modular, hierarchical local area network is realizable by using existing twisted pair wiring, for example, in a building together with several elements known as network access units and hub units. A network access unit is connected to a device (microprocessor, display terminal, peripheral device, other local area network, information systems network or the like) in the local area network to give that device access to and from the network. The network access device is designed for simple daisy chaining with other devices to form a node when devices are colocated in the same section or room in the building. The hub unit is the network building block which permits extensive expansion of the local area network. It provides a connection point for access units or other hub units, performs collision detection operations, if necessary, and serves as a loop-back point for the network. Switchably connectable termination impedances and loop-backs are automatically passed to the appropriate points in the local area network. While most small local area networks are considered for intra-building applications, the present network is applicable to inter-building applications, also.

This application is a continuation of application Ser. No. 714,683,filed Mar. 21, 1985, now abandoned.

TECHNICAL FIELD

This invention relates to the field of communication systems and, moreparticularly, to shared resource arrangements such as local areanetworks.

BACKGROUND OF THE INVENTION

A local area network is a high speed, multiple access, privatecommunication network. Depending upon the communication medium, forexample, cable, wire or optical fiber, the local area network has thecapability both to handle data at rates exceeding 10 M bps and toconnect more than 50,000 devices.

Local area networks are currently experiencing widespread acceptance asa means for providing interconnection and communications among hostcomputers, terminals and peripheral equipment located throughout asingle building or group of buildings. These local area networks haveemerged from a variety of different architectures based on thepoint-to-point, star, ring, bus and tree topologies.

Point-to-point topology causes each device in the network to beconnected to every other device. As such, point-to-point topology islimited to small networks because of high associated costs ofimplementation and difficulty in monitoring network activity due to thelack of a central communications control device.

Star topology is popular because it affords centralized control of thenetwork from a single switching node to which all devices in the localarea network are connected. In this topology, device connections aresimple and can often be accomplished using existing wire.

Ring topology is one in which each device is connected to exactly twoother devices in the local area network. A message token is passed toeach device in sequence around the ring in order to permit each devicean opportunity to send or receive information. While this topologyreduces considerably the number of device connections over thepoint-to-point and star topologies, it does not offer centralizedcommunications control. Moreover, when a device in the networkexperiences a failure, the entire local area network may be disableduntil the failed device is removed or replaced.

Bus topology is characterized by the plurality of devices in the networkbeing connected individually to a single, bidirectional, broadbandcommunications medium referred to as a bus. Generally, each device has aunique address and can gain bus access by a centralized or distributedinterrupt priority scheme. Message collisions are avoided through theuse of specific protocols. Tree topology resembles several bus networkslinked together via a common bus. This topology is applicable tonetworks which must operate over long distances.

As stated above, several different media can be used to carry local areanetwork communications. Considerations regarding network topology,maximum distance between nodes, volume of information to be transmitted,and speed of transmission are critical in selecting a particularcommunications medium. Physical limitations such as plenum, conduitsizes, and routing plans in the building also affect the choice of themedium. Finally, for some network topologies, user accessibility to thebus for passive tapping is extremely important.

Commercially available local area networks such as Ethernet (Ethernet isa trademark of Xerox Corporation) make use of the various topologies andtransmission media described above. At the present time, coaxial cableappears to be the medium of choice because it is capable of supportingvery high bit rates. However, local area networks relying on a coaxialcable transmission medium require the use of complex repeaters andregenerators to maintain signal integrity and to obtain reasonabletransmission distances for the network. Coaxial cable is also quiteexpensive and bulky which presents problems when wiring a local areanetwork in an existing building. Also, since many present local areanetworks involve some type of bus structure and because the busstructure is physically long, there are many signal timing andsynchronization problems caused by dispersive and reflective effects ofthe transmission medium which must be overcome by the collisiondetection circuitry and bus coupler or tap spacing, at the very least.Finally, many of the present local area networks lack modularity andmodular interconnectivity.

SUMMARY OF THE INVENTION

Overcoming these and other problems and in accordance with theprinciples of the present invention, a modular, hierarchical local areanetwork is realizable by using existing twisted pair wiring, forexample, in a building together with several elements known as networkaccess units and hub units. A network access unit is connected to adevice (microprocessor, display terminal, peripheral device, other localarea network, information systems network or the like) in the local areanetwork to give that device access to and from the network. The networkaccess device is designed for simple daisy chaining with other devicesto form a node when devices are colocated in the same section or room inthe building. The hub unit is the network building block which permitsextensive expansion of the local area network. It provides a connectionpoint for access units or other hub units, performs collision detectionoperations, if necessary, and serves as a loop-back point for thenetwork. Switchably connectable termination impedances and loop-backsare automatically passed to the appropriate points in the local areanetwork.

BRIEF DESCRIPTION OF THE DRAWING

A more complete understanding of the invention may be obtained byreading the following description of a specific illustrative embodimentof the invention in conjunction with the appended drawing in which:

FIG. 1 is a simplified block diagram of a daisy chain nodeconfiguration;

FIG. 2 is a simplified block diagram of a daisy/star configuration for alocal area network;

FIG. 3 is a simplified block diagram of an extended daisy/starconfiguration for a local area network;

FIG. 4 shows, in simplified schematic form, a circuit diagram for anetwork access unit in accordance with the principles of the invention;

FIG. 5 shows, in simplified schematic form, a circuit diagram for a hubunit; and

FIG. 6 shows, in schematic form, a receiver circuit, a logical ORfunction bus and a collision detection circuit integrated as in thereceiver portion of a hub unit.

DETAILED DESCRIPTION

Various possible configurations for local area networks designed inaccordance with the principles of this invention are shown in FIGS. 1through 3. Detailed diagrams of local area network elements shown simplyas blocks in FIGS. 1 through 3 are given in FIGS. 4 through 6.

Network elements (hub units and access units) bearing the same name butdifferent reference numerals are understood to be substantiallyidentical unless expressly stated to the contrary. It is understood bythose skilled in the art that the configurations and diagrams shown inthe Figures are merely for purposes of illustration rather thanlimitation.

Overview of Feature for Present Local Area Network

The following network features are present for the local area networkconfigurations shown in FIGS. 1-3 when deployed using the networkelements shown in FIGS. 4-6. Existing building wiring, both from servicecloset to service closet and from service closet to each office in thelocal area network, is used to interconnect network elements. Rewiringusing coaxial cable or the like is unnecessary.

Use of existing wiring also provides other benefits such as ease ofconfigurability of the network and ease of interconnection to thenetwork. Since a transmission medium already exists from an office tothe service closet, one need only connect a network access unit to theoffice end of the transmission medium and a hub unit to the servicecloset end of the same medium in order to configure the networkoffice-by-office. Interconnection is simplified because most modernwiring schemes make use of modular connector jacks and sockets toterminate the transmission medium.

Repeaters and/or regenerators are not needed in the present network toboost the transmitted signal power and resynchronize the transmittedsignal in order to complete a round trip through the network.Unrepeatered transmission is achieved, in one exemplary network, bystandard line drivers and receivers for differentially balanced signaltransmission on dual twisted pair wire. The present local area networkcan be extended over a large geographical area by the use of repeatersand regenerators.

Separate transmission paths are used for broadcasting signals to thenetwork and for receiving signals therefrom.

The present network access unit connected to each device (end-user) inthe local area network permits simple daisy chaining from one device tothe next. While close proximity is preferred between adjacent daisychained devices, it is not required. Also, within each daisy chain,interconnection of network access units automatically causes bothimpedance termination of the transmission medium to be connected at onlyone end ("far end") of the medium and short circuit or loop-back fromone transmission path to the other to be connected at the opposite end("head end").

Network management and control are accomplished without the use ofsophisticated electronic switches or private branch exchangescentralized in the network. Rather, the collision detection or avoidancemechanisms disposed throughout the network provide a suitable degree ofmanagement and control.

The present hub unit, which interconnects a group of network accessunits together, includes bus structures for each separate transmissionpath. These bus structures are the only busses in the local areanetwork. Each bus is extremely short, for example, either the backplanefor a printed circuit card rack or the input to an integrated circuitsuch as a multiple input NAND or OR gate. As such, these busses are moreaccurately called virtual busses. Because the busses are virtual(physically short), group delay and other signal distortion processes donot affect the integrity of the signals on the busses. Therefore, thereis no need for retiming/delaying various signals on the busses.

Also, the present hub unit includes a collision detection arrangement,if necessary, connected to the bus structures between the separatetransmission paths. Detection is performed by relying on and examiningknown characteristics of digital signals on the bus rather thancomposite, corrupted analog signals.

Loop-back is also provided by the hub unit in a similar manner to thatprovided by the network access unit. The loop-back connection isdisabled (and passed to the next higher hub in a hierarchical sense)when one hub is connected to another hub.

In the present local area network, in accordance with this invention,termination impedances are located at each endpoint whereas only oneloop-back point is located at the highest point, hierarchically, in thelocal area network.

These and other features of the present invention will become apparentto those persons skilled in the art upon reading the followingdescription.

DESCRIPTION OF LOCAL AREA NETWORK CONFIGURATIONS

FIG. 1 shows a basic building block of the local area network, that is,a daisy chain node. Daisy chain node 100 includes N devices connectedtogether in daisy chain fashion via N access units individuallyassociated with each device as shown. Each access unit provides inputand output connection for its associated device to and from thetransmission paths as well as providing input and output ports to alloweasy extension of or connection to the daisy chain in either direction.Daisy chaining refers to the fact that each device, through its accessunit, is effectively connected in parallel across the transmissionmedium which, in FIG. 1, is shown as dual balanced twisted pair wire.Each device includes a circuit arrangement (not shown in the Figures)which is capable of monitoring data received by the device to determinewhether a collision has been detected between two or more signals on thetransmission medium.

Specifically, device 101-1 has its output connected via lead 110-1directly to access unit 120-1; the output of access unit 120-1 isconnected via lead 112-1 to device 101-1. As shown in FIG. 1, each ofthe remaining N-1 devices is connected with its associated access unitin a similar manner to that described above.

Interconnection from one device to the next is facilitated by dualtwisted pairs of wires (four wires) connected between the output/inputport of one access unit and the input/output port of the next adjacentaccess unit. As shown in FIG. 1, dual twisted pair 130-1 is connectedbetween the output/input port of access unit 120-1 and the input/outputports of access unit 120-2. Similar connections are made via dualtwisted pairs (dual twisted pair 130-2 through dual twisted pair130-(N-1)) to the remaining access units (through access unit 120-N) indaisy chain node 100.

In order to provide a good transmission path in daisy chain node 100,each transmission line in the dual twisted pair must be terminated by animpedance equivalent to the characteristic impedance of the line. Thecharacteristic impedance terminations are generally located at the "farend" of the transmission path, for example, at the input/output port ofaccess unit 120-1 connected to terminal 121-1 through terminal 124-1.Effective communication is provided by a simple "loop-back" or shortfrom one twisted pair to the other. This normally occurs at the "headend" of the transmission path, for example, at the output/input port ofaccess unit 120-N connected to terminal 125-N through terminal 128-N. Asan alternative to externally terminating and looping back theinput/output and output/input controllable ports of the respectiveaccess units, switchable termination impedances and short circuits maybe provided as an internal circuit feature of each access unit whereinconnection of a dual twisted pair to an input/output or output/inputport automatically disconnects the termination impedance and shortcircuit loop-back connections, respectively, within that access unit.This feature will be discussed in more detail below with respect to FIG.4.

Throughout the description above, it has been assumed that N devicescomprise a daisy chain node. Clearly, N is an integer which is greaterthan or equal to 1.

FIG. 2 shows a local area network configuration referred to as adaisy/star configuration. "Daisy" refers to the fact that there is aplurality of daisy chain nodes; "star" refers to the fact that theconfiguration of daisy chain nodes and hubs grouped into clustersresembles a star having hub unit 250 as its center and cluster 201-1through cluster 201-K as its endpoints.

The K daisy chain nodes in cluster 201-1 individually resemble daisychain node 100 in FIG. 1. Within cluster 201-1, each daisy chain node isconnected to hub unit 230 via a dual twisted pair attached to theoutput/input port of one access unit (typically the "far end" accessunit) in the daisy chain node. This latter connection is illustrated bydual twisted pair 130-N in FIG. 1. Specifically, daisy chain node 210-1is connected to hub unit 230 via dual twisted pair 220-1. The remainingK-1 daisy chain nodes in cluster 201-1 are connected to hub 230 bysimilar dual twisted pairs 220-2 through 220-K. For completeness, itshould be noted that K is an integer greater than or equal to 1 and isrelated to the number of input/output ports available in a hub unit.

The hub unit provides several important features for the local areanetwork. First, each hub unit permits interconnection via K hubinput/output ports for K groupings of devices wherein a grouping iseither a cluster or a daisy chain node. Second, each hub unit provides alogical bus structure for signals to be communicated among devices.Finally, each hub includes, when applicable, appropriate collisiondetection apparatus to avoid interference among devices trying tocommunicate simultaneously. It should be clear to those persons skilledin the art that collision detection apparatus is unnecessary when tokenpassing or other collision-free protocols or other electronic orhardwire protocols are employed with the daisy/star configuration or anyother configuration designed in accordance with the principles of thisinvention.

In addition to the features described above, each hub provides a singlehub output/input port which serves either to connect one hub unit toanother or to provide a loop-back point for the network similar to thatof the daisy chain "head end." Hub-to-hub connection is shown from hubunit 230 to hub unit 250 via dual twisted pair 240-1. Similarconnections occur between hub units in the remaining K-1 clusters andhub unit 250. Loop-back occurs internally or externally at theoutput/input port of hub 250, specifically at hub terminal 251-1 throughhub terminal 251-4.

FIG. 3 shows a further extension of the daisy/star local area networkconfiguration in which a plurality of daisy/star networks are connectedto a single hub. In particular, daisy/star network 301-1 (as shown byFIG. 2) is connected from its hub output/input port (terminals 251-1through 251-4) to the hub input/output port of hub unit 350 via dualtwisted pair 310-1. Similar connection is made between the remaining K-1daisy/star networks and input/output ports of hub unit 350. Theoutput/input port of hub unit 350 at hub output terminal 351-1 throughhub output terminal 351-4 serves as a loop-back point (as describedhereinabove) for the extended daisy/star local area network.

The local area network architectures shown in FIGS. 1 through 3 are notmeant to be all inclusive. The architectures are hierarchical in natureand can be expanded accordingly. For example, devices includemicrocomputers, display terminals, printers, PBXs, switching devices,information service networks, other local area networks and the like.The devices may be connected directly to each other as a daisy chainnode or they may be individually connected to a hub unit anywhere in thelocal area network. Groups of devices or daisy chain nodes whichterminate to a hub unit form a cluster. Groups of clusters combined atanother hub unit form a daisy/star network. Plural daisy/star networkscombine at a hub unit to form an extended daisy/star network, and so on.Clearly, many combinations, permutations and extensions are possiblebased upon the principles incorporated in these Figures. While eachnetwork architecture has been explored and described with a certaindegree of symmetry, there is no reason militating against asymmetricconstruction of the local area network. Reduced to its most simpleterms, this means that a hub unit located anywhere in the network mayserve as a connection point for either daisy chain nodes or clusters ordaisy/star networks or any combination of the above.

Dual twisted pairs have been described above as a preferred transmissionmedium. Several reasons for this preference are readily apparent andbased upon efficiency, economic concerns and transmissioncharacteristics. For example, intra-building and, particularly,inter-office telephone wiring is generally accomplished in advance ofany local area network planning by using multiple twisted pair cables.Hence, the medium providing point-to-point network interconnection isalready in place for existing buildings. Access to the cable pairs isusually simplified by the use of modular connector jacks and sockets.This wire is small in size and offers ease in installation, cutting,splicing and testing. From a safety standpoint, the twisted pairpresents a substantially reduced shock hazard over shielded coaxialcable because all power and data signals are transformer coupled. Also,it is significantly easier to match the characteristic impedance oftwisted pair wire than matching that of coaxial cable. Finally, thetwisted pair, from a transmission line standpoint, offers a reasonabledegree of noise immunity especially to other services such as voicecircuits sharing the same cable binder group.

While dual twisted pair interconnections have been described above, itshould be obvious to those persons of ordinary skill in the art thatother transmission media can be substituted for the dual twisted pairs.For example, dual untwisted (ribbon type) pair wire may be substitutedfor the twisted pair transmission medium. Also, a singletwisted/untwisted pair of wires can be employed wherein one wire servesas a transmission path away from a device and the other lead serves as areturn path toward the same device. Because such a substitution resultsin an unbalanced transmission condition, access units and hub units mustbe correspondingly changed from differentially balanced logic elementdesign to unbalanced logic element design. Examples of other suitabletransmission media include dual optical fibers (single mode ormultimode), dual coaxial cable, a single bidirectional,polarization-preserving, optical fiber. Incorporation of the lattertransmission media in the local area network requires that correspondingchanges be made, where necessary, to the access units and hub units.

DESCRIPTION OF LOCAL AREA NETWORK ELEMENTS

FIG. 4 shows a simplified circuit diagram for a local area networkaccess unit. Access unit 120-1 includes an input/output port includingterminals 121-1 through 124-1, an output/input port including terminals115-1 through 118-1, line driver 140, line receiver circuit 141,switches 150 through 153, termination impedances 170 and 171, andtransmission paths 160 through 163. As described above, the access unitis connected to its associated device through leads 110-1 and 112-1.

Switches 150 through 153 are normally closed. If any media connectorsuch as a dual twisted pair is connected to the input/output port,switches 150 and 151 via the connection made to terminals 121-1 and124-1, respectively, are electrically, mechanically, or otherwise openedthereby removing the termination impedances 170 and 171 from thiscircuit. Similarly, when connection of a dual twisted pair is made tothe output/input port via terminals 115-1 through 118-1, the shortcircuit or loop back is removed from this particular circuit becauseswitches 152 and 153 are caused to be opened. Switches or other devicescapable of performing in this manner in conjunction with a terminalconnection are well known throughout the telephone and associatedindustries. Such devices distributed by AT&T are jacks from the 657J and657S families and mating plug assemblies. It should be understood that,while the state of each switch is shown to be controlled by connectionto particular terminals of the access unit (shown by the dashed lines inthe Figure), the state of each switch is functionally related to whetherthere is a connection to the particular port.

Transmission paths 160 through 163 couple the input/output port atterminals 121-1 through 124-1, respectively, directly with theoutput/input port at terminals 115-1 through 118-1, respectively,thereby maintaining the daisy chain structure.

When no connection is made to the input/output port of the access unit,termination impedance 170 terminates transmission path 160 and 161 andtermination impedance 171 properly terminates paths 162 and 163. Thetermination impedances are selected to approximate and match thecharacteristic impedance of the transmission paths.

When operating in a dual twisted pair interconnection environment asdepicted in the Figures, line driver 140 is a differentially balancedline driver. An input signal from the associated device on lead 110-1 istranslated into a pair of balanced signals. Each balanced signal is thenapplied either directly or through impedance matching transformers orother similar impedance matching devices to its respective transmissionpath, either path 160 or 161. Assuming that no connection is made to theoutput/input port, then it is apparent that the balanced signal on path160 is shunted directly through switch 153 to path 162. In a similarmanner, the balanced signal on path 161 is shunted directly throughswitch 152 to path 163.

Line receiver circuit 141 receives the balanced signals fromtransmission paths 162 and 163 to reconstruct an unbalanced input signalto be supplied to the associated device via lead 112-1. Line receivercircuit 141 is realizable as a standard differentially balanced signalline receiver or as a simple zero crossing detector or as a more complexcombination of an energy detector controlling a zero crossing detectoras shown in FIG. 6.

FIG. 5 shows one embodiment for hub unit 250 in a more detailedschematic circuit form. The hub unit includes K input/output ports, asingle output/input port, K hub port circuits, an optional collisiondetection circuit and an access unit. These elements cooperate toprovide the generic hub unit functions described above.

Each input/output port is access via a set of four terminals. Forexample, the K-th port is accessed via terminals 501-K through 504-K.The output/input port, usually a loop-back or network extension point,is attached via terminals 251-1 through 251-4.

Hub port circuit 500-1 is comprised of both a receive portion includingtermination impedance 505, line receiver circuit 506 and transmissionpath 507 and a transmit portion including transmission path 508 and linedriver 504. the remaining K-1 hub port circuits are substantiallyidentical to hub port circuit 500-1. Termination impedance 505 connectedbetween terminals 501-1 and 502-1 is then chosen to match thecharacteristic impedance of the transmission path. Line receiver circuit506 provides balanced-to-unbalanced signal conversion identical to linereceiver 141 (see FIG. 4). Transmission path 507, as shown in FIG. 5, isone embodiment of a logical OR bus when extended through all K hub portcircuits.

The logical OR bus formed by the series connection of receive portiontransmission paths 507 from each hub port circuit is connected by lead520 to either collision detection device 510 or via the optional shortcircuit (dashed line) to lead 521 and access unit 511.

Collision detection device 510, when used, monitors the signals suppliedvia lead 520. If it appears that two signals have collided, that is,interfered with each other, then collision detection device 510 inhibitsfurther transmission of the interfering signals and broadcasts onto lead521 by a predetermined signal that a collision has occurred. Uponreceipt of such a broadcast, each device in the network ceasestransmission.

Access unit 511 is identical to the access unit shown in FIG. 4. Theoutput/input port of access unit 511 acts as the output/input port forthe hub unit. Signals supplied to access unit 511 on lead 521 aretranslated to differentially balanced signals on terminal 251-1 andterminal 251-2. Loop-back or other balanced signals arriving atterminals 251-3 and 251-4 are converted to unbalanced signals by theaccess unit 511 and supplied to the series connected hub port circuitson lead 522.

The transmission path 508 associated with the transmit portion of eachhub port circuit is serially connected to the corresponding transmissionpaths 508 in adjacent hub port circuits. Signals on transmission path508 are converted by line driver 504 into differentially balancedsignals available at terminals 503-1 and 504-1.

FIG. 6 shows an alternative embodiment of the receive portion of a hubunit. This embodiment includes K line receiver circuits 600-1 through600-K, a logical OR bus structure, and collision detection circuit 611.

Each line receiver circuit includes an energy detector and a zerocrossing detector, both of which have their output signals gatedtogether. In line receiver circuit 600-1, balanced input signals arereceived from terminals 501-1 and 502-1 of the corresponding hubinput/output port. The balanced input signals are shared between theenergy detector 601 and the zero crossing detector 609. In energydetector 601, the balanced signals are each offset by a predetermined dcvoltage represented by dc sources 602 and 603. The offset balancedsignals are then applied to a line receiver 604 to convert the offsetbalanced signals to an unbalanced signal. This unbalanced signal issupplied to a rectifier and low pass filter section comprised of diode605, resistor 606 and capacitor 607. The diode, resistor and capacitorare chosen to cause a fast attack and slow decay energy detectionprocess. The attack time is limited by the internal impedance of linereceiver 604. This limited attack time prevents narrow noise transientsfrom asserting energy detection. Thus, energy detection is asserted forsignals which exceed the offset bias for a time period greater than theattack time of the diode, resistor and capacitor combination. In anexample from experimental practice, the resistor and capacitor valuesare chosen to cause the attack time to be on the order of one-fourthperiod of a data bit and the decay time to be on the order of several(two or three) bit periods. The output of the low pass filter is asmooth unbalanced signal referred to as energy detector signal EDl.Signal EDl is supplied to the input of NAND gate 608.

The line receiver circuit also includes zero crossing detector 609 whichis realizable as a line receiver. The unbalanced signal output from zerocrossing detector 609 is supplied to NAND gate 608. When the energydetector determines that a valid signal is present, NAND gate 608 isactivated to transmit the output signal from zero crossing detector 609.When an invalid signal is present or in the absence of a signal asdetermined by the energy detector, NAND gate 608 is disabled. The linereceiver circuits 600-1 through 600-K generate line receiver outputsignals LR1 through LRK, respectively.

Line receiver output signals LR1 through LRK are supplied to K-inputNAND gate 610. Gate 610 operates as a logical OR bus structure (invertedlogic) providing a single output to collision detection circuit 611. Thepath from the output of gate 610 to circuit 611 is equivalent to thelead 520 in FIG. 5. Logic gate 610 can be changed to be an OR gateprovided accomodating logical inversions and other changes are made.

Collision detection circuit 611 includes multiplexer 612, logic array613 and collision presence oscillator 614. The collision detectioncircuit determines if more than one incoming signal is present at the Kinput/output ports of the hub unit and, on the basis of thatdetermination, it either allows the output of NAND gate 610 to be passedto the access unit or transmits a collision presence signal whichnotifies the entire local area network that a collision has occurred.

In operation, logic array 613 receives the energy detector outputsignals ED1 through EDK from all line receiver circuits in the hub unit.Logic array 613 is configured to output a logically high energyviolation signal EV only when the K energy detector output signalsindicate the presence of two or more energy detector signals being high.Otherwise, energy violation signal EV is in a low state. When signal EVis high, it causes the collision presence oscillator 614 to be activatedand it causes multiplexer 612 to output the error signal supplied by thecollision presence oscillator. Otherwise, when signal EV is low, thecollision presence oscillator is inactive and multiplexer 612 outputsthe signal supplied from NAND gate 610.

In an example from experimental practice, unbalanced signals in thenetwork are Manchester encoded baseband signals transmitted at a rate ofapproximately 1.0 Mbps. As such, collision presence oscillator 614generates a signal which produces a large number of Manchester codeviolations within a predetermined interval. For example, the signal fromcollision presence oscillator 614 could be as simple as a 0.667 MHz.signal which produces an optimum number of severe Manchester codeviolations within a given information bit period. In this case, thereare extreme variations between zero crossings of the Manchester codedsignal at 1 Mbps and the 2/3 MHz signal.

What is claimed is:
 1. A local area network arrangement for providingcommunications on first and second segmented transmission means among aplurality of devices, each device communicating through a network accessmeans connected to the device and to the first and second segmentedtransmission means, each network access means including,transceivermeans for translating signals from the device to the first segmentedtransmission means via first and second ports and for translatingsignals from the second segmented transmission means via first andsecond ports to the device, the first port including input terminalmeans for connecting the first segmented transmission means thereto,output terminal means for connecting the second segmented transmissionmeans thereto, and means for terminating the first and second segmentedtransmission means each with a characteristic termination impedance, theterminating means being connected from the input terminal means of thefirst port to the output terminal means of the first port only in theabsence of connection between the segmented transmission means and thefirst port, and the second port including input terminal means forconnecting the second segmented transmission means thereto, outputterminal means for connecting the first segmented transmission meansthereto, and loop back means connected from the input terminal means tothe output terminal means of the second port for providing a lowimpedance electrical path therebetween only in the absence of aconnection between the segmented transmission means and the second port,the input and output terminal means of the first port being connecteddirectly to the output and input terminal means, respectively, of thesecond port, the second port of each network access means adapted forconnection via the segmented transmission means to the first port of anadjacent network access means.
 2. The local area network arrangement asdefined in claim 1 wherein the first segmented transmission means is atwisted pair of wires and the second segmented transmission means is atwisted pair of wires.
 3. The local area network arrangement as definedin claim 1 wherein the local area network is further comprised of atleast first hub means for expanding the local area network by providinginterconnectivity between other such hub means and network access meansvia first and second segmented transmission means, the at least firsthub means including,first and second common bus means, first throughK-th ports, each port having both input terminal means for connectingthe first segmented transmission means thereto and output terminal meansfor connecting the second segmented transmission means thereto, firstthrough K-th port circuit means, each port circuit means being connectedto the corresponding port for translating signals from the inputterminal means to the first common bus means and for translating signalsfrom the second common bus means to the output terminal means, an accessmeans connected in circuit relationship to the first and second commonbus means for translating signals from the first common bus means to thefirst segmented transmission means and for translating signals from thesecond segmented transmission means to the second common bus means, anda (K+1)th port including input terminal means for connecting the secondsegmented transmission means thereto, output terminal means forconnecting the first segmented transmission means thereto, and loop backmeans connected from the input terminal means to the output terminalmeans of the (K+1)th port for providing a low impedance electrical paththerebetween only in the absence of a connection between the segmentedtransmission means and the (K+1)th port.
 4. The local area networkarrangement as defined in claim 3 wherein the first segmentedtransmission means is a twisted pair of wires and the second segmentedtransmission means is a twisted pair of wires.
 5. The local area networkarrangement as defined in claim 3 wherein the at least first hub meansfurther includes collision detection means connected to the first commonbus means and the access means for determining the occurrence of acollision between two or more signals on the first common bus means andsupplying a collision signal representing such collision occurrence tothe access means and, in the absence of signal collision, for permittingtransmission of the signal on the first common bus means to the accessmeans.
 6. The local area network arrangement as defined in claim 5wherein the first segmented transmission means is a twisted pair ofwires and the second segmented transmission means is a twisted pair ofwires.
 7. The local area network arrangement as defined in claim 5wherein the collision detection means is connected in the circuitbetween the first common bus means and the access means.
 8. The localarea network arrangement as defined in claim 7 wherein the firstsegmented transmission means is a twisted pair of wires and the secondsegmented transmission means is a twisted pair of wires.
 9. The localarea network arrangement as defined in claim 8 wherein the collisiondetection means, upon detection of signal collision on the first commonbus means, inhibits transmission of colliding signals to the accessmeans.
 10. The local area network arrangement as defined in claim 5wherein the first common bus means is a K-input logic element capable ofperforming a logical OR function on the signals from first through K-thport circuit means.
 11. The local area network arrangement as defined inclaim 10 wherein the first segmented transmission means is a twistedpair of wires and the second segmented transmission means is a twistedpair of wires.
 12. A network access arrangement for use in a local areanetwork providing communications on first and second segmentedtransmission means among a plurality of devices, each device connectedthrough a corresponding network access arrangement connected to thedevice and to the first and second transmission means, said networkaccess arrangement including,transceiver means for translating signalsfrom the device to the first segmented transmission means via first andsecond ports and for translating signals from the second segmentedtransmission means via first and second ports to the device, the firstport including input terminal means for connecting the first segmentedtransmission means thereto, output terminal means for connecting thesecond segmented transmission means thereto, and means for terminatingthe first and second segmented transmission means each with acharacteristic termination impedance, the terminating means beingconnected from the input terminal means of the first port to the outputterminal means of the first port only in the absence of connectionbetween the segmented transmission means and the first port, and thesecond port including input terminal means for connecting the secondsegmented transmission means thereto, output terminal means forconnecting the first segmented transmission means thereto, and loop backmeans connected from the input terminal means to the output terminalmeans of the second port for providing a low impedance electrical paththerebetween only in the absence of a connection between the segmentedtransmission means and the second port, the input and output terminalmeans of the first port being connected directly to the output and inputterminal means, respectively, of the second port, the second port ofeach network access arrangement adapted for connection via the segmentedtransmission means to the first port of an adjacent network accessarrangement.
 13. The network access arrangement as defined in claim 12wherein the first segmented transmission means is comprised of a twistedpair of wires and the second segmented transmission means is comprisedof a twisted pair of wires.
 14. A hub arrangement for use in a localarea network providing communications on first and second segmentedtransmission means among a plurality of devices, each device connectedthrough a corresponding network access arrangement connected to thedevice and to the first and second transmission means, the local areanetwork further comprised of at least one of said hub means, said hubmeans providing an expansion capability to the local area network byproviding interconnectivity between other such hub means and networkaccess means via first and second segmented transmission means, said hubmeans including,first and second common bus means, first through K-thports, each port having both input terminal means for connecting thefirst segmented transmission means thereto and output terminal means forconnecting the second segmented transmission means thereto, firstthrough K-th port circuit means, each port circuit means being connectedto the corresponding port for translating signals from the inputterminal means to the first common bus means and for translating signalsfrom the second common bus means to the output terminal means, an accessmeans connected in circuit relationship to the first and second commonbus means for translating signals from the first common bus means to thefirst segmented transmission means and for translating signals from thesecond segmented transmission means to the second common bus means, anda (K+1)th port including input terminal means for connecting the secondsegmented transmission means thereto, output terminal means forconnecting the first segmented transmission means thereto, and loop backmeans connected from the input terminal means to the output terminalmeans of the (K+1)th port for providing a low impedance electrical paththerebetween only in the absence of a connection between the segmentedtransmission means and the (K+1)th port.
 15. The hub arrangement asdefined in claim 14 wherein the first segmented transmission means iscomprised of a twisted pair of wires and the second segmentedtransmission means is comprised of a twisted pair of wires.
 16. The hubarrangement as defined in claim 14 wherein the at least first hub meansfurther includes collision detection means connected to the first commonbus means and the access means for determining the occurrence of acollision between two or more signals on the first common bus means andsupplying a collision signal representing such collision occurrence tothe access means and, in the absence of signal collision, for permittingtransmission of the signal on the first common bus means to the accessmeans.
 17. The hub arrangement as defined in claim 16 wherein the firstsegmented transmission means is comprised of a twisted pair of wires andthe second segmented transmission means is comprised of a twisted pairof wires.
 18. The hub arrangement as defined in claim 16 wherein thecollision detection means is connected in the circuit between the firstcommon bus means and the access means.
 19. The hub arrangement asdefined in claim 18 wherein the first segmented transmission meansiscomprised of a twisted pair of wires and the second segmentedtransmission means is comprised of a twisted pair of wires.
 20. The hubarrangement as defined in claim 19 wherein the collision detectionmeans, upon detection of signal collision on the first common bus means,inhibits transmission of colliding signals to the access means.
 21. Thehub arrangement as defined in claim 16 wherein the first common busmeans is comprised of a K-input logic element capable of performing alogical OR function on the signals from first through K-th port circuitmeans.
 22. The hub arrangement as defined in claim 21 wherein the firstsegmented transmission means is comprised of a twisted pair of wires andthe second segmented transmission means is comprised of a twisted pairof wires.